The invention relates to a vehicle provided with
at least three wheels,
a frame having a first frame section and a second frame section, the frame sections being tiltable with respect to one another about a tilt axis located in a longitudinal direction,
at least one front wheel which is connected to the first frame section and is rotatable about a front wheel steering shaft,
a steering wheel rotatably connected to the first frame section,
tilting means for tilting the first and the second frame sections relative to one another, and
a signal transmitter coupled to the tilting means in order, when driving round a bend having a predetermined bend radius, to generate a control signal for controlling the tilting means as a function of said bend radius.
A vehicle of this type is disclosed in WO 95/34459. This publication describes a self-balancing, preferably three-wheel vehicle having an active tilting mechanism, wherein the front frame section is tilted by means of hydraulic tilting cylinders when driving round a bend. In order to control the degree of tilt, the force or the moment on the front wheel is measured by a sensor, which in this case consists of a hydraulic rotary valve. In response to the signal from the sensor, the front frame section, which includes the driver""s cab and the steering wheel, is tilted until the force or the moment on the front wheel becomes virtually zero. In this way a self-stabilising tilting mechanism is obtained which makes the vehicle stable when taking a bend at any speed. Since narrow vehicles as described in WO 95/34459 in general have too low a lateral stability and manoeuvrability to be able to participate in normal (car) traffic, a tilting system of this type increases the stability such that a narrow vehicle of this type can be a satisfactory means of transport. The tilting system described is fully automatic, as a result of which the driver is not expected to have any special skills for steering the vehicle. The known system provides a safe vehicle with a predictable response in all driving conditions to be anticipated.
When the driver turns into a sharp bend rapidly by exerting a moment on the steering wheel, a moment directed towards the inside of the bend will have to be exerted on the tiltable frame section in order to provide the desired tilting. The tiltable frame section will tend to tilt towards the outside, which tendency has to be counteracted by the tilting cylinders. Especially in the case of rapid manoeuvres in which the driver pulls hard on the steering wheel, the front wheel will be turned into the bend to a considerable extent and the moment on the tilting frame section which is directed towards the outside of the bend will be high, so that the tilting cylinders will have to exert a high moment in order to provide the desired tilting. As a result of this the maximum tilting speed is restricted.
It is therefore an aim of the present invention to provide a tiltable vehicle of improved manoeuvrability. In addition to an increase in the tilting speed, it is also an aim of the invention to provide a vehicle with which tilting means can be of relatively compact construction.
To this end the vehicle according to the present invention is characterised in that the tilting means comprise an opposite steer power transmitter which is connected to the front wheel and to the frame, which opposite steer power transmitter exerts a moment on the front wheel that is directed away from the center of curvature of the bend. By means of the power transmitter according to the present invention, an active front wheel servomechanism is created which compensates for the moment which is exerted by the driver on the front wheel and produces slight turning of the front wheel towards the outside. As a result, the front frame section of the tilting vehicle will xe2x80x9cdrop into the bendxe2x80x9d more rapidly and the moments which have to be exerted by the tilting means on the front frame section in order to cause this to tilt are substantially reduced. If the power transmitter according to the invention is used in a tilting vehicle having an active tilting mechanism, that is to say where, when taking a bend, the front frame section is tilted with respect to the rear frame section by means of a separate drive device, the drive device to produce the tilting can be of relatively small construction as a result of the use of the opposite steer power transmitter. If the drive device comprises hydraulic cylinders, said cylinders can be small and/or operate at lower pressure.
The opposite steer power transmitter according to the present invention can, however, also be used in vehicles without an active tilting mechanism, tilting being produced solely by the power transmitter.
The vehicle according to the present invention can have a tiltable frame which is as described in WO 95/34459 and which is provided with three or four wheels, or can comprise a tiltable frame as described in Netherlands Patent Application number 1 005 894. Furthermore, the vehicle according to the invention can include tiltable elements in the wheel suspension, wherein the first frame section is made up of the wheel axles and the second frame section is made up of the parts supported by the wheel axles, such as, for example, the driver""s cab. In this case the wheel axles can remain in a constant position when the second frame section is tilted with respect to the road surface.
The invention is based on the insight that there are two ways of changing the degree of tilt of a vehicle: 1. by exerting a tilting moment, as a result of which the vehicle is actively tilted towards the inside of the bend, and 2. by a steering manoeuvre on the front wheels in the opposite direction to the desired tilt direction, as a result of which the vehicle will tend to lean towards the inside of the bend like a motorbike.
The use of the first method alone has the disadvantage that the tilting moment that can be exerted is restricted by the characteristics (width, mass of the tiltable frame section with respect to the fixed frame section) of the vehicle, as a result of which the achievable tilting speed (and thus the manoeuvrability) are limited. Moreover, a steering moment (towards the inside of the bend) exerted by the driver will cause the tiltable frame section to lean towards the outside, as a result of which part of the available tilting moment will already be used to prevent the tiltable frame section from dropping down further towards the outside. As a consequence the maximum tilting speed is reduced, possibly even to zero.
This can be improved by the opposite steer power transmitter according to the invention, which is connected to the front wheel/the front wheels and the frame section to which the front wheels are attached. Depending on the layout of the vehicle, said section does not have to be the tilting frame section. Said opposite steer power transmitter preferably exerts a steering moment on the front wheel(s) which is a function of (for example proportional to) and in the opposing direction to the tilting moment exerted between the tilting and the non-tilting frame section of the vehicle.
As a result of said steering moment on the front wheels, specifically directed towards the outside of the desired bend, the vehicle will tilt towards the inside more rapidly than in the absence of said power transmitter. In combination with an active tilting system, the maximum tilting speed of the vehicle will be appreciably increased. The steering moment that is exerted by the power transmitter is preferably proportional to the tilting moment and will decrease as the position comes closer to the desired tilt position.
Preferably, the vehicle according to the present invention has an active tilting mechanism in the form of a drive device which is connected to the first and the second frame section in order to exert a tilting moment between the first and the second frame section. With this arrangement the signal transmitter for controlling the opposite steer power transmitter can, in a preferred embodiment, be incorporated in said drive device, so that the tilting moment that is exerted on the tilting frame section is used as a control signal for the opposite steer power transmitter. The absence of a tilting moment on the frame sections means that the vehicle is in the correct tilt position when taking a bend and that the opposite steer power transmitter does not have to be activated. If there is a tilting moment on the front and rear frame sections, this means that the tilt position has to be modified when taking the bend and specifically has to be modified in the direction of the tilting moment. Said tilting of the frame sections can be effected both by the drive device and by the opposite steer power transmitter. When the opposite steer power transmitter is controlled by the tilting moment between the frame sections, the amplification factor is not critical: if the amplification factor is low, little opposite steer will be produced by the opposite steer power transmitter and the vehicle will tilt largely as a consequence of the active tilting produced by the drive device. If the amplification factor is very high, the front wheel will apply a large amount of opposite steer because of the opposite steer power transmitter, so that tilting of the vehicle will be produced mainly by application of said opposite steer and to a lesser degree by the action of the drive device. The front wheel will, however, never apply too great an opposite steer since in that case the vehicle would lean too far into the bend, with the result that the tilting moment on the frame sections would become negative and the operation of the opposite steer power transmitter, and consequently the application of opposite steer by the front wheel, would be terminated immediately.
Irrespective of the way in which the driver""s input is translated into a tilt position and driving direction, it is possible to construct the signal transmitter for the opposite steer power transmitter as, for example, a xe2x80x9clateral acceleration sensorxe2x80x9d which is mounted in the tilting frame section. Said sensor measures the error in the position of the tilting frame section. By using this signal to control the opposite steer power transmitter, the latter will actively steer the front wheel and reduce the error in the tilt position to zero. As a consequence the primary tilting system can be of simpler and more compact construction. Additional safety can also be built in by this means, for example by triggering a fitted emergency system or warning signal if a specific tilt position error angle is exceeded. In this case the signal from the acceleration sensor is a suitable input signal for the opposite steer power transmitter with an amplification factor to be chosen.
Any system can be used to control the drive device for active tilting of the frame sections, for example the system as described in EP-A 0 592 377. With this system the drive device for tilting the front frame section relative to the rear frame section, which carries the drive, is obtained by angular rotation of the front wheel about the front wheel axle with respect to the frame. EP-A 0 020 835 describes a tilting vehicle wherein the drive device for tilting the frame sections is controlled by means of foot pedals or by a movement of the steering column, transversely to the axis of rotation thereof. Preferably, however, the drive device is controlled by a sensor which generates a signal that is dependent on the force or moment exerted on the front wheel. This type of control of the drive device is described in WO 95/34459. In a further preferred embodiment, the steering wheel is connected via a steering shaft to the first frame section, the steering shaft being rotatable with respect to the front wheel about its center line. The drive device is controlled by a turning angle sensor which measures an angle of rotation between the front wheel and the steering shaft. A construction of this type is described in International Patent Application number PCT/NL98/00534.